In operating a conventional pneumatic tool such as an air-operated hammer or chisel for breaking concrete, cutting stone, digging road surface or doing other percussion works, the tool unit held in the front end portion of a pneumatic tool will be actuated to percuss the working object as reciprocatively impacted by a piston reciprocatively driven in the barrel of a pneumatic tool. When piercing through a working object by the tool unit or when the tool unit is slipped from the working object, the shoulder of the tool unit may be relatively separated from the piston to cause an "empty percussion" of the piston, thereby increasing vibrational shock hazard and influencing occupational safety and health.
If the pneumatic tool is designed to automatically shut off its compressed air supply to temporarily stop the reciprocative movements when the tool is under no load (empty percussion) condition, the vibration hazard of the pneumatic tool will then be prevented or reduced, and the benefit of the occupational safety and health can then be obtained.